1. Field of Invention
The present invention relates to an apparatus for driving a liquid crystal display panel, a liquid crystal display apparatus, and an electronic apparatus and, more particularly, to an apparatus and method for driving an active matrix drive type of a liquid crystal display panel using a two-terminal type non-linear element such as a MIM (metal insulator metal) element having a bidirectional diode characteristic, to a liquid crystal display apparatus provided with the driving apparatus (liquid crystal display module), and to an electronic apparatus provided with the liquid crystal display apparatus.
2. Description of Related Art
Conventional active matrix drive type of liquid crystal display panels include those using a two-terminal type non-linear element such as a MIM element having a bidirectional diode characteristic, as well as those using TFT (thin-film transistor) elements. MIM elements, etc., have a sharper change at a threshold and therefore have the advantage of being simple in structure and being manufactured by a simpler process in comparison with TFT elements.
FIG. 15 shows a liquid crystal display panel using a MIM element as the above-mentioned two terminal type non-linear element. This liquid crystal display panel is constructed in such a manner that, as shown in FIG. 15, a liquid crystal layer and a MIM element layer are connected in series to form one pixel region at each of intersections of a plurality of data signal lines ( . . . , Xi-1, Xi, Xi+1 . . . ) and a plurality of scanning signal lines ( . . . , Yj-1, Yj, Yj+1 . . . ), which are respectively arranged on a pair of substrates so as to form a matrix. A scanning signal drive circuit 81 is connected to the scanning signal lines while a data signal drive circuit 82 is connected to the data signal lines. From the scanning signal drive circuit 81, a scanning signal is supplied to each scanning signal line. From the data signal drive circuit 82, a data signal is supplied to each data signal line. In each pixel region, therefore, the MIM element can be driven in an on-off manner if the potential difference between the scanning signal and the data signal at the pixel region is set in a certain level relationship with the threshold voltage of the MIM element. When the MIM element is turned on, the liquid crystal layer connected to the MIM element is charged to turn on the pixel region. After charging for a predetermined time period, the MIM element is turned off and set in a high-impedance state. This state of the MIM element and the resistance of the pixel region set to a sufficiently large value enable the charge on the liquid crystal layer to be retained, thereby maintaining the on state of the pixel region. As the time for selecting and charging one of the pixel regions (hereinafter referred to as "selecting period"), a part of the time period through which the pixel region is maintained in the on state may suffice, as described above. Therefore, this selecting period can be set with respect to each of the scanning signal lines in a time division manner, thus enabling matrix drive of the plurality of pixel regions sharing the scanning signal lines and the data signal lines.
As a typical example of such a drive method, a drive method called four-value drive method can be mentioned. A four-value drive method is a method of using a two-value scanning signal and a two-value data signal and inverting the polarities of the scanning signal and the data signal about the middle value of the data signal with the passage of every horizontal period, for example, and inverting the polarities about the middle value of the data signal with the passage of every vertical period with respect to each scanning signal line. This method can be practiced with a comparatively simple circuit arrangement.
As mentioned above, the liquid crystal display panel using the MIM element is constructed so that the MIM element and the liquid crystal layer are connected in series in each pixel region. Accordingly, the voltage applied to the liquid crystal layer immediately after each selecting period depends upon the voltage applied to the MIM element at the corresponding time. The voltage applied to the MIM element at the corresponding time, i.e., the voltage applied to the MIM element when charging of the liquid crystal layer is stopped substantially completely, depends upon the current-voltage characteristic of the MIM element. Therefore, an error can occur in the voltage applied to each of the MIM elements due to a variation in the current-voltage characteristics of the MIM elements. Such a voltage error cannot be canceled out in the four-value drive method or the like, in which the polarities of scanning and data signals are simply inverted about the middle value of the data signal with the passage of every vertical period to simply invert the polarity of the voltage applied to the liquid crystal layer. Consequently, an error of the above-described kind occurs between the pixel regions to cause a variation in the voltage applied to the liquid crystal layer in each pixel region, which results in a display unevenness or the like.
A drive method called charge and discharge method has been proposed as a drive method for achieving an improvement in display characteristics in comparison with the four-value drive method. This drive method comprises making a MIM element conductive by charging and making the MIM element conductive by discharging after overcharging opposite in polarity to the charging about the middle value of a data signal, and is arranged for driving in charging and discharging modes, as shown in FIGS. 16(B) and 17(C). In the charging mode, a first selecting voltage (VS1) is supplied to one scanning signal line to charge the liquid crystal layer by the voltage of the potential difference from a data signal. On the other hand, in the discharging mode, a voltage-VPRE which is a precharge voltage opposite in polarity to the first selecting voltage (VS1) about the middle value of the data signal is supplied to overcharge the liquid crystal layer. Successively, a second selecting voltage (VS2) opposite in polarity to the precharge voltage about the middle value of the data signal is supplied to discharge the overcharged liquid crystal layer. Therefore, if the amount of discharge is controlled by the data signal during the time period through which the second selecting voltage (VS2) is supplied, the displaying state of the pixel region can be controlled.
For example, if, as shown in FIG. 16(A), a data signal having values VH/2 and -VH/2 is supplied to data signal line Xi in every horizontal period (the period indicated by 1H in FIG. 16(A)), and if, as shown in FIG. 16(B), a scanning signal having a selecting potential such as that described above is supplied to scanning signal line Yj, voltage VB1 applied to the liquid crystal layer in the pixel region at the intersection of data signal line Xi and scanning signal line Yj immediately after the end of the selecting period in the charging mode is given by the following equation. EQU VB1=(VS1+VH/2-VON)-K.multidot.(VS1-VH/2) (1)
wherein K is a capacitance ratio expressed as CM/(CM+CL) if the capacitance of the MIM element is CM and the capacitance of the liquid crystal layer is CL; K.multidot.(VS1-VH/2) represents a shift of the liquid crystal layer voltage caused through capacitive coupling at the moment when the MIM element is turned off; and VON is the voltage applied to the MIM element when charging of the liquid crystal layer is stopped substantially completely.
In the discharging mode, after overcharging at precharge voltage-VPRE, the accumulated charge is discharged by the second selecting voltage VS2, so that the voltage applied to the liquid crystal layer after the end of the selecting period is VS2-VH/2-VON. Accordingly, the voltage VB2 applied to the liquid crystal layer immediately before the end of the selecting period is shown by the following equation. EQU VB2=(VS2-VH/2-VON)-K.multidot.(VS2-VH/2)=-{(VON-VS2+VH/ 2)+K.multidot.(VS2-VH/2)} (2)
wherein K.multidot.(VS2-VH/2) represents a shift of the liquid crystal layer voltage caused through capacitive coupling at the moment when the MIM element is turned off, as in the case of the charging mode.
As is apparent from the above equations (1) and (2), if the voltage VON applied to the MIM element becomes higher by DVON when charging of the liquid crystal layer is stopped substantially completely, the absolute value of VB1 becomes smaller by DVON and, conversely, the absolute value of VB2 becomes larger by DVON. On the other hand, if VON becomes smaller by DVON, the absolute value of VB1 becomes larger by DVON but the absolute value of VB2 becomes smaller by DVON. Further, if an error DK occurs in K, and if the absolute value of VB1 becomes larger by this error, the absolute value of VB2 becomes smaller. If the absolute value of VB1 becomes smaller by this error, the absolute value of VB2 becomes larger.
Thus, according to the charge and discharge drive method, even if VON of the MIM element changes, an error voltage caused in the liquid crystal applied voltage in the charging mode is canceled out, in terms of effective voltage, by an error voltage caused in the liquid crystal applied voltage in the discharging mode. Consequently, it is possible to effectively prevent occurrence of a display unevenness or the like due to a variation in VON of the MIM elements in the liquid crystal display panel.
The drive method shown in FIG. 16, however, has a drawback in that crosstalk of data can occur easily. For example, if a data signal having values such as shown in FIG. 17(A) is supplied to data signal line Xi and if scanning signals having values such as shown in FIGS. 17(B) and 17(C) are supplied to scanning signal lines Yj-1 and Yj, each of voltages of a waveform such as shown in FIG. 17(D) is applied between the opposite ends of the MIM element and the liquid crystal layer in pixel region (Xi, Yj) at the intersection between data signal line Xi and scanning signal line Yj. As shown in FIG. 17(D), the voltage applied between the opposite ends of the MIM element and the liquid crystal layer in overcharging period Tdcj in the overcharging mode is -VPRE-VH/2. This is because the scanning signal supplied to scanning signal line Yj has voltage-VPRE and the value of data signal kDi,j-1 supplied to data signal line Xi in this period is VH/2. Letter k in the symbol for the data signal denotes the field number, the suffix i attached to the symbol D for symbolization of data denotes the number of the data signal line, and the suffix j-1 denotes the number of the scanning signal line. That is, data signal kDi,j-1 is a data signal for the pixel region corresponding to the intersection of data signal line Xi and scanning signal line Yj-1 when the kth field is formed.
In discharging period Tdj for the pixel region (Xi, Yj), data signal kDi,j for the pixel region (Xi, Yj) is supplied to enable the voltage between the opposite ends of the MIM element and the liquid crystal layer to be set to the desired value. However, the above-described voltage between the opposite ends of the MIM element and the liquid crystal layer in overcharging period Tdcj depends upon the value of data signal kDi,j-1 supplied to the pixel region at the intersection of scanning signal line Yj-1 and data signal Xi one line before, resulting in occurrence of crosstalk. Such a crosstalk can occur because each of overcharging period Tdcj and discharging period Tdj is set in one horizontal period.
To eliminate such a crosstalk, a drive method such as shown in FIG. 18 has been proposed. FIG. 18(A) shows a data signal supplied to data signal line Xi, and FIG. 18(B) shows a scanning signal supplied to scanning signal line Yj-1. Also, FIG. 18(C) shows a scanning signal supplied to scanning signal line Yj, and FIG. 18(D) shows voltages applied between the opposite ends of the MIM element and the liquid crystal layer in pixel region (Xi, Yj) at the intersection between data signal line Xi and scanning signal line Yj.
In this drive method, as shown in FIG. 18(A), a data signal is supplied only in the second half of one horizontal period while the ground potential is supplied in the first half period. As shown in FIG. 18(B), a period for charging with the scanning signal in the charging mode is also set in the second half of one horizontal period. As shown in FIG. 18(C), overcharging period Tdcj in the overcharging mode is set in the first half of one scanning period, and discharging period Tdj is set in the second half of the one scanning period.
The voltage applied between the opposite ends of the MIM element and the liquid crystal layer in overcharging period Tdcj is thereby made-VPRE independent of the data signal value, as shown in FIG. 18(D), thus preventing crosstalk of the data signal.
In the drive method shown in FIG. 18, however, charging is performed only in the second half of one horizontal period in the charging mode and overcharging is performed in the overcharging mode only when the data signal value is at the ground potential. Therefore, a sufficient voltage cannot be applied to the liquid crystal layer, resulting in a reduction in contrast.
With respect to this problem, it is conceivable that the peak to peak amplitude of the scanning signal is increased to compensate for a deficiency of the charging voltage. However, if the applied voltage is increased, the MIM element exhibits a saturated characteristic such that the amount of charge in the overcharging period in the overcharging mode is limited. Therefore, this drive method is not substantially effective in solving the problem of low contrast.
Also, the absolute value of precharge voltage VPRE of the scanning signal in the overcharging period may be increased. However, the increase in this precharge voltage VPRE is limited because of the withstand voltage performance of the liquid driver provided in the scanning signal drive circuit 81.